A conventional signal transmission insulating device having a thin film transformer structure is known in which a first insulating film is formed by forming a lower coil on the bottom of a recess provided on a semiconductor substrate, filling the recess with a liquid polyimide resin, and curing the liquid polyimide resin, an upper coil is formed on the first insulating film, and the thickness of the first insulating film is adjusted, thereby ensuring a desired insulation withstand voltage between the lower coil and the upper coil (see, e.g., Patent Document 1).
In such a signal transmission insulating device having a thin film transformer structure, when a voltage is applied to the upper coil and the lower coil, electric field concentration occurs at each corner of each of the upper coil and the lower coil. The signal transmission insulating device having a transformer structure has a plurality of corners, and insulation breakdown occurs from any of the corners of the upper coil or the lower coil when the applied voltage increases. Meanwhile, as in a signal transmission insulating device disclosed in Patent Document 2, in a signal transmission insulating device in which a second insulating film having a higher permittivity than a first insulating film is provided on a surface of a lower coil which surface faces an upper coil and the first insulating film and the upper coil are sequentially formed on the second insulating film, the second insulating film, which has a higher permittivity than the first insulating film, is formed at the lower coil side between the lower coil and the upper coil so as to be in contact with the lower coil. Thus, an electric field within the second insulating film becomes small, and electric field concentration at each corner of the lower coil, which is in contact with the second insulating film, can be alleviated.